Users of obsolete mainframe computers running a proprietary operating system may have a very large investment in proprietary application software and, further, may be comfortable with using the application software because it has been developed and improved over a period of years, even decades, to achieve a very high degree of reliability and efficiency.
As manufacturers of very fast and powerful commodity processors continue to improve the capabilities of their products, it has become practical to emulate the proprietary operating systems of powerful older computers such that the manufacturers of the older computers can provide new systems which allow the users to continue to use their highly-regarded proprietary software by emulating the older computer.
Accordingly, computer system manufacturers are developing such emulator systems for the users of their older systems, and the emulation process used by a given system manufacturer is itself subject to ongoing refinement and increases in efficiency and reliability.
Some historic computer systems now being emulated by commodity processors have non-conventional word lengths. Most, perhaps all, commodity processors have word lengths which are a power of two, and, at the state of the art, 64-bit word lengths are widely used. However, some historic computer systems now being emulated by commodity processors have non-conventional word lengths. An example is the Bull HN Information Systems (descended from General Electric Computer Department and Honeywell Information Systems) GCOS-8 (General Comprehensive Operating System) operating system which employs a 36-bit word. The historic reason for the adoption of this word length was to permit data storage of four eight-bit characters per word, each character also being provided with a ninth, parity, bit to improve error-checking and fault tolerant capabilities and also to provide more precision than that provided by competing systems employing shorter word lengths such as 32 bits.
In one specific state-of-the-art example, a 64-bit Itanium Intel processor is used to emulate the Bull DPS (Distributed Processor System) 9000 36-bit memory space and the GCOS 8 instruction set of the DPS 9000. Within the memory space of the emulator, the 36-bit word of the DPS 9000 is stored right justified (least significant bits) in the least significant 36 bits of the “host” 64-bit word. The upper 28 bits of the 64-bit word are typically zero for “legacy” code. Sometimes, certain specific bits in the upper 28 bits of the containing word are used as flags or for other temporary purposes, but in normal operation these bits are usually zero and in any case are always viewed by older programs in the “emulated” view of the world as being non-existent. That is, only the emulation program itself uses these bits.
For some purposes, such as providing new or more direct communication with programs or services running in the 64-bit system, it would be advantageous to provide the emulated system with full access to the entire “containing” word, which in this case is the 64-bit containing word, for purposes of both loading or storing the 64-bit word from the viewpoint of the emulated software into visible space within the 36-bit environment. The capability of viewing and manipulating 64 bits can also be used to improve the machine architecture by such expedients as adding new opcodes with more functionality, increasing the address space, or other similar things which can utilize more bits in the instruction or data words.
The present invention is directed to achieving this end.